Iris Vs Shutter Speed

By John Honovich, Published Jan 24, 2013, 12:00am EST

Surveillance cameras must deal with wide variations of lighting -- automatically and continuously. It can be 10,000+ lux during the day and under 1 lux at night. In a few seconds, lighting can drop dramatically, for example, if the sun goes behind a cloud or if a person turns off the lights. As lighting changes, the camera needs to adjust on its own without having a photographer optimize the scene.

Surveillance cameras do this through two means - controlling the size of the iris or the speed of the shutter. Indeed, often cameras can control both simultaneously. In this tutorial, we compare the two and explain how they relate.

For background, see our iris tutorial and shutter speed tutorial.

Controlling Light Input

Cameras are like "Goldilocks" - they cannot deal with too little or too much light, it needs to be just right.

exposure contrast

Of course, the amount of light that is 'just right' for a camera depends on how much light is in the scene. In the example above, if the blinds were open, the scene would have far more light and the camera would need to adjust to that .

Two "Controls"

Think of cameras and light like water in a pipe. If you want to control the water you can do it two ways:

  • Make the pipe bigger (or smaller); For cameras, this is adjusting the iris size
  • Open the pipe longer (or shorter); For cameras, this is adjusting the shutter speed

Why One Vs. the Other

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Some cameras (fixed or manual iris ones) rely solely on adjusting the shutter speed. There's no choice here.

Other cameras (auto or P iris ones) can make the iris size dynamically larger or smaller. In addition, typically, they will also be able to control the shutter speed as well.

One huge difference is that the shutter speeds are nearly limitless while iris has a hard physical limitation. One can only open the iris so wide until the lens physically cannot support anything more. However, the shutter can be made to stay open for as long as the camera manufacturer (and user) permits. While this will allow far more light and a far brighter picture in dark conditions, it can also create massive blurring that makes moving objects useless.

On the other hand, controlling the iris can allow for a deeper field of view. However, this has limited practical benefit in surveillance and will not work in dark conditions because the higher F stops required to do this will make the image dark and useless. See our Depth of Field tutorial.

Practical Impact

Key take-aways. Remember that:

  • Cameras need to automatically and continuously regulate the amount of light that enters
  • This can be done by adjusting the width of the opening for light (iris) or the length of time light enters (shutter)
  • Shutter provides great flexibility and capability for letting in lots of light but with the risk of blurring
  • Iris provides the potential for deeper FoV but it is neither necessary nor practical for most surveillance applications
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